Disable BasicTests/VFS on win32 for now. Investigating.

  Failing Tests (5):
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.CaseInsensitive
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.MappedFiles
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.MultiComponentPath
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.TrailingSlashes
    Clang-Unit :: Basic/BasicTests.exe/VFSFromYAMLTest.UseExternalName